Free commercial email services such as Gmail, Yahoo!, and Outlook Web Mail (formerly Hotmail) are free for a reason: they are vehicles for tracking your interests and serving up advertisements. If a service is free, you are the product, not the customer; a good example is Facebook.com.
One way to prevent eavesdropping on your email is to use encrypted email. Microsoft Office 365, Intermedia, and several other paid email systems offer this ability at a reasonable cost.
Most of us could care less about the eavesdropping, however, there are those that didn't know about the fine print in the terms and conditions of these free email providers they accepted.